Remove explicit sslMode disable from default PostgreSQL storage config

What does this MR do?

Removes the explicit sslMode: "disable" default from values.yaml for the PostgreSQL storage backend. OpenBao uses the pgx driver; when sslMode is omitted, pgx defaults to prefer, attempting TLS first and falling back to plaintext if the PostgreSQL server does not support TLS. Users can explicitly set sslMode: "disable" to preserve plaintext-only connections.

• Sets the values.yaml default for sslMode to null, with schema type and enum annotations.
• Documents the pgx prefer default and upgrade option in values.yaml.
• Regenerates values.schema.json.
• Updates tests/configmap_test.yaml to verify sslmode is omitted by default and renders when explicitly configured.
• Bumps chart version to 0.20.2 (main is now 0.20.1).

Author\x27s Checklist

Required

• Ensure a release milestone is set.
• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline on GitLab.com.
• When ready for review, MR is labeled workflowready for review.

Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated (values.yaml includes the default and upgrade guidance; GitLab chart docs are updated in MR !5468, which is blocked by this MR).
• Tests are added.

Related issues

Closes #40 (closed)